This nonprovisional application claims priority under 35 U.S.C. § 119(a) on Patent Application No(s). 091137905 filed in TAIWAN, R.O.C. on Dec. 30, 2002, which is(are) herein incorporated by reference.
1. Field of the Invention
The invention generally relates to a synthesis method of composite nanofibers, and particularly relates to a method for synthesizing composite nanofibers by forming a second nanofiber inside a first hollow nanofiber that plays as a secondary template.
2. Related Art
Recently, nanotechnology is extremely hot in industries. Many breakthroughs are obtained and undoubtedly cause great impacts to the industry. Among numerous nano-scale materials, nanofibers have excellent characteristics in their energy and photoelectric properties so as to be highly noticed.
A general method for producing nanofibers is the vapor deposition for fabricating vapor-growth carbon fibers. A carbon fiber is a hollow tubular structure having a diameter of 5˜20 nanometers and having an outer surface on which a porous high surface area layer is formed. The porous surface makes the nanofiber an excellent adsorbent and catalyst support. However, the fabrication process is costly and energy intensive that limits the production and applications.
In view of this limitation, cost-oriented manufacturing process is an important point of nanofiber fabrication. For example, template synthesis is a method for producing high quality and lower cost nanofibers and taking the place of the expensive vapor deposition.
Different template synthesis methods have been developed for nanofiber fabrication. For example, sol-gel for SiO2, SnO2, V2O5, etc; electroless plating for Nickel; electro-deposition for ZnO, and so on. Specific template synthesis methods are applied in accordance with the materials and applications. However, a single material nanofiber usually cannot meet the application requirements. For example, in the application of lithium-ion secondary batteries, the Martin research group found that SnO2 nanofibers for negative pole material of a lithium cell, though having a high reversible electric capacity larger than 700 mAh/g and high current discharge rate of 58 C, has a high irreversible electric capacity that limits the applications. The irreversible electric capacity is caused by a solid-electrolyte interphase of Li2O formed from deoxidation of SnO2 and lithium-ions. The high irreversible electric capacity increases the surface impedance and decrease the lifetime of the nanofibers. The reaction mechanism is shown in FIG. 5. The reactions are as follows:4Li++4e−+SnO2→2Li2O+Sn  (equation I)xLi++xe−+Sn←→LixSn, 0≦x≦4.4  (equation II)Wherein equation I shows the formation of Li2O; equation II shows the reversible reaction of Li—Sn alloy, which provides the reversible electric capacity.
Therefore, as shown in FIG. 6, if a suitable material, such as a carbon coating, is applied on surface of a single material nanofiber, such as tin oxide SnO2, for inhibiting the formation of solid-electrolyte interphase and decreasing the irreversible electric capacity, then the applicability of nanofibers can be improved.
The concept of synthesizing composite nanofibers for overcoming the problem of irreversible electric capacity in lithium battery application is thus generated. However, though the fabrication of single material nanofiber is easier, when forming a second material coating on exterior of the first nanofiber through conventional chemical vapor deposition or chemical impregnation, the coating is uneven in thickness and hard to be obtained. Therefore, bi-material nanofiber with even composition is a great difficulty of fabrication with conventional processes.